Ion exchange resin membrane is in wide use as a membrane for cell (e.g. polymer electrolyte fuel cell, redox flow cell or zinc-bromine cell), a membrane for dialysis, etc. Polymer electrolyte fuel cell uses an ion exchange resin membrane as the solid electrolyte membrane. When a fuel and an oxidant are fed continuously into this polymer electrolyte fuel cell, they react with each other, generating a chemical energy. The chemical energy generated is taken out as an electric power, and the fuel cell is one of power generation system which is clean and highly efficient.
In recent years, the polymer electrolyte fuel cell has increased its importance for uses in automobile, household and portable devices because it can be operated at low temperatures and can be produced in a small size.
The membrane-catalyst electrode assembly (hereinafter, this may be abbreviated as membrane-catalyst electrode assembly) incorporated in the polymer electrolyte fuel cell generally comprises an ion-exchange resin membrane acting as an electrolyte and diffusion electrodes bonded to each side of the membrane. Each diffusion electrode is generally constituted by a porous electrode material and a catalyst-loaded electrode bonding to the porous electrode material. When an electric power is taken out from the polymer electrolyte fuel cell, hydrogen gas or a liquid fuel (e.g. methanol) is fed into a chamber (a fuel chamber) in which one of the two diffusion electrodes is present, and an oxygen-containing gas (e.g. oxygen or air) as an oxidant is fed into a chamber (an oxidant chamber) in which the other diffusion electrode is present. When, in this state, an external load circuit is connected to the two diffusion electrodes, the fuel cell works as such and an electric power is supplied to the external load circuit.
Of polymer electrolyte fuel cells, direct liquid fuel cell utilizing methanol or the like for fuel, is evaluated highly because it uses a liquid fuel easy to handle and the fuel is inexpensive. For these reasons, the direct liquid fuel cell is expected as a power source of relatively small output which is used for portable devices.
The fundamental structure of polymer electrolyte fuel cell is shown in FIG. 1. In FIG. 1, 1a and 1b are each a partition wall of cell. The cell partition walls 1a and 1b are formed at the both sides of a solid polymer electrolyte membrane 6 made of an ion-exchange resin membrane, so as to sandwich the solid polymer electrolyte membrane 6. The solid polymer electrolyte membrane 6 functions as a membrane.
2 is a fuel passage formed in the inner wall of one cell partition wall 1a, and 3 is an oxidant gas passage formed in the inner wall of other cell partition wall 1b. 4 is a diffusion electrode of fuel chamber side, and 5 is a gas diffusion electrode of oxidant chamber side.
In using a cation-exchange electrolyte membrane as the solid polymer electrolyte membrane 6, when a liquid fuel (e.g. alcohol) or a gaseous fuel (e.g. hydrogen) is fed into a fuel chamber 7, protons (hydrogen ions) and electrons are generated by the action of the catalyst provided in the fuel chamber side diffusion electrode 4. The protons pass through the inside of the solid polymer electrolyte membrane 6 and reach an oxidant chamber 8, where the protons react with the oxygen in air or in oxygen gas, generating water. Meanwhile, the electrons generated at the fuel chamber side diffusion electrode 4 pass through an external load circuit (not shown) and are sent to the oxidant chamber side gas diffusion electrode 5. At this time, an electric energy is supplied to the external load circuit.
In the polymer electrolyte fuel cell having the above-mentioned structure, there is ordinarily used a cation-exchange resin membrane as the solid polymer electrolyte membrane 6. On the surface of the cation exchange resin membrane are formed diffusion electrodes 4 and 5. Ordinarily, hot pressing is used for formation of the diffusion electrodes 4 and 5. In this hot pressing, first there is formed, on a substrate, a diffusion electrode constituted by a porous electrode material and a catalyst electrode layer formed on one side thereof. Then, the diffusion electrode is heat-transferred from the substrate onto the surface of a cation-exchange resin membrane. The cation-exchange resin membrane and the catalyst electrode layer are made into one piece by the thermal compatibilization of the polymer electrolyte binder impregnated into the catalyst electrode layer and the cation-exchange resin consisting the cation-exchange resin membrane.
A perfluorocarbonsulfonic acid resin membrane has been used most typically as the cation-exchange resin membrane used as a membrane for fuel cell. However, the following problems are pointed out for the cation-exchange fuel cell using the perfluorocarbonsulfonic acid resin membrane.    (i) Since the field of reaction is strongly acidic, only a noble metal catalyst is usable.    (ii) The perfluorocarbonsulfonic acid resin membrane is expensive and there is a limit in cost reduction.    (iii) Since the physical strength of the resin membrane is low, it is difficult to reduce the electrical resistance of the resin membrane by making thin the resin membrane.    (iv) The resin membrane is low in water retention.Accordingly, it is necessary to supplement water in order to maintain the proton conductivity of the resin membrane.    (v) When methanol is used as the fuel, the permeability of methanol through the resin membrane is high and methanol reaches the gas diffusion electrode of oxidant chamber side, where methanol reacts with oxygen or air at the catalyst surface of the diffusion electrode, generating an overvoltage. As a result, a reduction in output voltage takes place (the same occurs also when other liquid fuel is used).
In order to solve these problems, it is being actively investigated to use, in place of the perfluorocarbonsulfonic acid resin membrane, a hydrocarbon cation-exchange membrane. However, the above problem (i) has not been solved by using any of such hydrocarbon cation-exchange membranes.
Hence, in order to solve the above problems, particularly the problem (i), it is being investigated to use, in place of the perfluorocarbonsulfonic acid resin membrane, a hydrocarbon anion-exchange membrane; and several proposals have been made (see, for example, Patent Literatures 1 to 3). In the fuel cell using an anion-exchange membrane, the field of reaction is basic and the risk of catalyst corrosion is low. Therefore, a catalyst other than noble metal is considered to be usable.
The mechanism in which a fuel cell using an anion-exchange membrane generates an electric energy, is described below. In this case, the ionic species moving inside the solid polymer electrolyte membrane 6 differs from the ionic species of the fuel cell using a cation-exchange membrane. That is, a liquid fuel (e.g. methanol) or a gaseous fuel (e.g. hydrogen) is fed into the fuel chamber side and oxygen and water are fed into the oxidant chamber side, whereby, in the oxidant gas diffusion electrode 5, the catalyst contained in the electrode contacts with the oxygen and the water, generating hydroxide ion (OH−). This hydroxide ion passes through the solid polymer electrolyte membrane 6 made of the above-mentioned hydrocarbon anion-exchange membrane and moves into the fuel chamber 7. The hydroxide ion reacts with the fuel at the fuel diffusion electrode 4, generating water. In this case, the electron generated at the fuel diffusion electrode 4 is sent to the oxidant gas diffusion electrode 5 via an external load circuit.
In the fuel cell using an anion-exchange membrane, the energy generated by the above reaction is utilized as an electric energy.
In the fuel cell using a hydrocarbon anion-exchange membrane, not only the above problem (i) but also the problems (ii) to (iii) can be greatly reduced generally.
In the direct liquid fuel cell using a liquid fuel (e.g. methanol), the problem (iv) is reduced by the water fed from a water-containing fuel. Further, it is expected that the problem (v) of methanol permeation through membrane can be considerably reduced for the following reason. That is, during the flow of electric current, hydroxide ion of large ionic diameter moves from the oxidant chamber side toward the fuel chamber side (the direction of this movement is opposite to the direction of methanol permeation). The movement of methanol is hindered by the above movement of hydroxide ion and is suppressed.
Besides, since the field of reaction is basic, the overvoltage of oxygen reduction at the diffusion electrode of oxidant chamber side can be lowered.
The polymer electrolyte fuel cell using a hydrocarbon anion-exchange membrane has such advantages. The hydrocarbon anion-exchange membrane incorporated into a fuel cell includes a membrane comprising a porous membrane (e.g. woven fabric) and a hydrocarbon type crosslinked polymer having an anion-exchange group (e.g. quaternary ammonium salt group or quaternary pyridinium salt group), filled in the porous membrane (Patent Literature 1); a membrane obtained by introducing a quaternary ammonium salt group into a hydrocarbon engineering plastic and then subjecting it to casting (Patent Literature 2); a membrane obtained by graft-polymerizing a hydrocarbon monomer having an anion-exchange group, on a substrate made of a fluorine-containing polymer; etc.
The formation of catalyst electrode layers 4 and 5 on the anion-exchange membrane (solid electrolyte membrane 6) is conducted in the same manner as in the formation on the cation-exchange membrane. That is, each catalyst electrode layer is formed using a coating fluid comprising an electrode catalyst, a binder made of an anion-exchange resin, and a solvent; then the catalyst electrode layer formed is bonded to an anion-exchange membrane by hot pressing. As the binder made of an anion-exchange resin, there is disclosed an anion-exchange resin obtained by aminating a chloromethylation product of a copolymer of aromatic polyethersulfone and aromatic polythioethersulfone (Patent Literatures 1 and 2).
The formation of catalyst electrodes can also be conducted by producing a catalyst electrode sheet made of an electrode catalyst and a polytetrafluoroethylene binder, coating thereon a binder made of an anion-exchange resin, and press-bonding the coated catalyst electrode sheet to an anion-exchange membrane to bond them to each other. As the binder, there is used a polymer obtained by treating the terminal of a perfluorocarbon polymer having a sulfonic acid group, with a diamine, for quaternization (Patent Literature 3).
Such an anion-exchange membrane has a high hardness because the membrane uses a reinforcing material or the resin constituting the anion-exchange membrane has a crosslinked structure so that the membrane can suppress the permeation of fuel or can give a mechanical strength. Or, the anion-exchange membrane uses a resin material of relatively high hardness (e.g. engineering plastic) for the same purpose. Further, as the binder made of an anion-exchange resin, used in formation of catalyst electrode layer, an engineering plastic of relatively high hardness is used. As the binder made of an anion-exchange resin, there is also used a resin whose main structure is a fluorocarbon resin low in compatibility with hydrocarbon anion-exchange membrane.
For these reasons, the adhesivity between the catalyst electrode layers 4 and 5 and the anion-exchange resin membrane is inferior and the adhesion at their interface tends to be poor. Consequently, the resistance at the interface is high. Further, the interface between the catalyst electrode layer and the anion-exchange resin membrane is exposed to a liquid fuel when used in a fuel cell. As a result, the adhesion strength at their interface tends to decrease. Further, each catalyst electrode layer and the anion-exchange resin membrane differ in chemical structure, composition, etc. and accordingly differ in the degree of swelling in liquid fuel. Therefore, poor adhesion tends to appear at the interface and peeling occurs ultimately between the anion-exchange membrane and the catalyst electrode layer.
In order to enhance the bondability between the hydrocarbon anion-exchange membrane and the catalyst electrode layer, there is a proposal of using, as the binder for catalyst electrode layer, an anion-exchange resin in which an anion-exchange group is introduced into a hydrocarbon polymer elastomer (Patent Literature 4). This literature discloses only a method for bonding, to an anion-exchange membrane, a catalyst electrode layer formed using the above-mentioned anion-exchange resin and an electrode catalyst, by thermal pressing, to bond them to each other. Their bondability, however, is not sufficient.
Patent Literature 1: JP-A-1999-135137
Patent Literature 2: JP-A-1999-273695
Patent Literature 3: JP-A-2000-331693
Patent Literature 4: JP-A-2002-367626